1. Field of the Invention
This invention relates to a process for controlling vibration damping force in a vibration damping device suitable for use in an automobile suspension system for passenger car, truck and the like, a suspension system for railroad vehicles, a vibration damper for industrial machines, a vibration absorbing device for buildings and the like.
2. Description of the Related Art
For instance, vibration damping devices comprising a closed fluid chamber divided into two parts by a partition member inclusive of a flexible member, an electrorheological fluid filled in the fluid chamber, a restricted passage disposed in the fluid chamber and electrodes arranged in the restricted passage are disclosed in U.S. Pat. No. 4,858,733 and Japanese Patent laid open No. 63-266237. In these vibration damping devices, when a direction of vibrational acceleration in an upper side portion of the device or a vibration output side portion thereof is the same as a direction of relative moving velocity between the vibration output side portion and a lower side portion or a vibration input side portion of the device, a high voltage is applied to the electrodes to increase the viscosity of the electrorheological fluid, whereby the vibration damping force is increased.
In the conventional technique of controlling the vibration damping force based on only the vibrational acceleration, however, when the device is used as an automobile suspension system, if a goading vibration resulted from the riding on protrusions is transmitted to the vibration input side portion of the device at such a state that the vibration damping force of the device becomes large, the device itself transmits a larger vibrating force to the vibration output side portion.